1. Field of the Invention
The present invention relates to an apparatus for calculating a quantity indicating the charged state (also called “internal charged state” or “internal state”) of a secondary (rechargeable) battery mounted on a vehicle, and in particular, to the apparatus that calculates the charged state using at least a plurality of pairs of voltage and current acquired from the on-vehicle battery.
2. Description of the Related Art
In general, it is required that the capacity and safety of on-vehicle battery systems be strictly managed. To perform such a management, the charged state of an on-vehicle battery system should be estimated with precision.
Several apparatuses to provide some techniques for the estimation have now been proposed. Such techniques use, for example, pseudo circuit-open voltage, internal resistance, charged rate, and/or remaining capacity. For example, as to detecting the internal resistance, there have been known Japanese Patent Laid-open Publication Nos. 2002-343444 and 2005-146939, in which a regression line is obtained on a current/voltage characteristic of a battery system to compute the amount of its slope as an internal resistance of the battery system. Another technique is provided by Japanese Patent Laid-open Publication No. 2002-168929. According to this publication, pairs of voltage/current of an on-vehicle battery are sampled during a period of time during which current consumed by a starter increases from zero to a peak value as high as some 1000 A in response to the start up of the starter, and then the acquired pairs of voltage/current are used to calculate the internal resistance of the battery in controlling the battery state.
In the period of time during which the starter-consuming current increases in response to the startup (hereinafter, this period is referred to as a “starter-current increasing period,” refer to FIG. 13), large changes in the current necessary for calculating the internal resistance with precision can be acquired in a short period of time. Thus, in this period of time, the influence of changes in the polarization of the battery in the calculation can be neglected, so that it may be expected that the internal resistance is detected with precision, with very small error due to polarization changes.
However, the foregoing starter-current increasing period is very short (for example, 2-5 msec). This means that a signal processing circuit to acquire a large number of pairs of voltage/current in such a short time should have a high signal processing performance. Specifically, this signal processing circuit includes a current sensor and an A/D converter for digitizing the current and voltage acquired, so that these circuit components need a higher sampling rate. To have such a high signal processing performance results in a rise in parts costs of the signal processing circuit.
In addition, a high-amplitude current which instantaneously reaches a value as high as 1000 A flows through the starter in the starter-current increasing period. The current sensor should therefore have a very large dynamic range, which also becomes. a factor in raising the parts costs.
On the other hand, it is inevitable that the fast-operating current sensor and the analog/digital signal processor are influenced largely by high-frequency components of radio noise and thermal noise, thereby lowering the S/N (signal to noise) ratio.
In this way, to produce voltage/current paired data in the starter-current increasing period involves a high S/N, a fast sampling performance and a wide dynamic range at the sensor and signal processing circuit. This is not practical as it burdens the circuit with a high operational performance. In other words, sampling voltage and current data during the starter-current increasing period is not practical.
To remove this difficulty, it may be possible to sample voltage and current data during a cranking period (or cranking time; refer to FIG. 13) following the starter-current increasing period in an engine startup period (refer to FIG. 13). In the cranking period, the engine starts to generate torque while the starter current decreases gradually. Hence using this cranking period, the burden of the sensor and circuit may be less.
However, since the cranking period is longer than the starter-current increasing period, the fact that changes in the polarized state of the battery during calculation of a charged state thereof cannot be negligible.